1. Field of the Invention
An aspect of the present invention relates to at least one of a calibration device, a calibration method, and a non-transitory computer-readable recording medium.
2. Description of the Related Art
Conventionally, a three-dimensional measurement technique based on a stereo method has been applied in a wide field. Herein, there are mainly a passive measurement method and an active measurement method for a three-dimensional measurement method based on a stereo method.
In an active measurement method, while a particular pattern image is projected onto a measurement target, the measurement target is imaged from a viewpoint different from a projection viewpoint and a correspondence between a projected pattern image and a pattern image on an imaged measurement target is made. Then, a distance to the measurement target is calculated by a principle of triangulation based on a position of a picture element at a corresponding point of each image, an internal parameter (such as a focal length, a position of an optical axis, or a lens distortion) of an optical system of each of a projection device and an imaging device, and an external parameter thereof (a relative position or an attitude of the projection device and the imaging device) (for example, Japanese Patent Application Publication No. 2011-141174).
Herein, preliminary acquisition of an internal parameter and an external parameter as described above prior to a three-dimensional measurement is referred to as a “calibration”.
In a calibration in an active measurement method, after a pattern image projected on a projection surface for calibration is normally imaged by an imaging device, a correspondence of the pattern image is made among a screen for calibration, a projection surface of a projection device, and an imaging surface of an imaging device, and based on a result thereof, internal and external parameters are acquired (for example, Japanese Patent Application Publication No. 2001-320652).
A precondition of such a calibration method is that it is possible for an optical system of a projection device to be approximated by a pinhole camera model (a central projection method in such a manner that projection light rays pass through one particular point), but a commercially available projection device may have an optical system differing from the pinhole camera model.
For example, Japanese Patent Application Publication No. 2008-165202 discloses a short focus projector using a concave mirror that does not condense projection light rays at one point in order to reduce degradation of an optical performance thereof at a lower cost, but it is not possible to apply a conventional calibration method to a three-dimensional measurement device using such a projector.